Collisions of electrons with atoms — AQA A-Level Physics

This topic explores what happens when free electrons collide with atoms, focusing on energy transfer and the quantised nature of atomic energy levels. You will learn how inelastic collisions can excite or ionise atoms, and how subsequent de-excitation leads to the emission of photons with discrete energies, explaining atomic line spectra.

Understanding these collisions is crucial because it provides experimental evidence for the Bohr model of the atom and underpins many areas of physics, including spectroscopy, lasers, and quantum mechanics. It also connects to the wider A-Level curriculum, such as the photoelectric effect and energy level diagrams.

• →Electrons in atoms can only exist at specific discrete energy levels; energy is quantised.
• →An incident electron can excite a bound electron to a higher level only if it transfers an amount of energy exactly equal to the energy level difference.
• →Ionisation occurs when the incident electron transfers enough energy to remove the bound electron from the atom completely.
• →In an inelastic collision, kinetic energy is conserved overall but some is transferred to the atom; in an elastic collision, no energy is transferred to the atom's internal energy.
• →De-excitation of an electron from a higher to a lower energy level results in the emission of a photon with energy E = hf = E_initial - E_final.

• Define ionisation as the removal of an electron from an atom, requiring energy greater than or equal to the ionisation energy.
• Define excitation as the promotion of an electron to a higher energy level within an atom, requiring a specific amount of energy.
• Explain that in a fluorescent tube, accelerated electrons collide with mercury atoms, causing excitation and ionisation.
• Describe how excited mercury atoms emit ultraviolet photons when they de-excite.
• Explain that the fluorescent coating absorbs ultraviolet photons and re-emits visible light.
• States that the electron volt is defined using the work done on an electron accelerated through a potential difference of 1 V.
• Uses W = QV with Q equal to the elementary charge 1.60 × 10⁻¹⁹ C and V equal to 1 V to derive 1 eV = 1.60 × 10⁻¹⁹ J.
• Applies the conversion factor correctly in either direction, multiplying by 1.60 × 10⁻¹⁹ to change eV to J and dividing to change J to eV.
• Recognises that the eV is an energy unit suited to atomic, nuclear and particle contexts, not a unit of potential difference or charge.
• Calculates the energy gained by a charged particle accelerated through a stated potential difference using E = QV and expresses it in eV or J.
• Recalls the conversion factor 1 eV = 1.60 × 10⁻¹⁹ J and applies it in the correct direction.
• Multiplies an energy in eV by 1.60 × 10⁻¹⁹ to obtain the energy in joules.
• Divides an energy in joules by 1.60 × 10⁻¹⁹ to obtain the energy in electron volts.
• Handles powers of ten correctly when converting values such as keV or MeV, for example 1 keV = 1.60 × 10⁻¹⁶ J.
• Presents the converted value with an appropriate unit and a sensible number of significant figures.

• 💡Use clear diagrams to show energy levels and transitions when explaining excitation and ionisation.
• 💡Link the processes step by step: acceleration of electrons, collisions, excitation/ionisation, emission of UV, absorption by coating, emission of visible light.
• 💡Remember that ionisation energy is the minimum energy needed to remove an electron from an atom, and it is specific to each element.
• 💡Write the conversion factor as 1 eV = 1.60 × 10⁻¹⁹ J and show the multiplication or division explicitly so the method is visible.
• 💡Check the magnitude of your answer: atomic-scale energies are usually a few eV, which is of order 10⁻¹⁹ J to 10⁻¹⁸ J.
• 💡When a question gives a potential difference, use E = QV with Q as the particle charge, then convert to eV by dividing by 1.60 × 10⁻¹⁹ J eV⁻¹ if needed.
• 💡Write the conversion as a clear equation, such as E(J) = E(eV) × 1.60 × 10⁻¹⁹, before substituting numbers.
• 💡Estimate the expected order of magnitude first so an incorrect power of ten is easy to spot.
• 💡Give the final answer in the unit requested by the question and include the unit symbol to avoid losing credit.
• 💡Always use the term 'exact energy level difference' when describing excitation, and specify that collisions are inelastic when energy is transferred to the atom.
• 💡When calculating photon wavelengths, show clearly that you use E = hf and c = fλ, and convert between eV and J correctly.
• 💡In explanations, structure your answer logically: describe the collision, energy transfer, excitation, de-excitation, and photon emission.

• Confusing ionisation with excitation. Correction: ionisation removes an electron completely, while excitation moves an electron to a higher energy level within the atom.
• Thinking that any collision energy can cause excitation. Correction: excitation requires a specific energy equal to the difference between energy levels.
• Believing that the mercury vapour emits visible light directly. Correction: mercury atoms emit ultraviolet photons, which are converted to visible light by the fluorescent coating.
• Treating the electron volt as a unit of potential difference; correct this by stating that the volt is the potential difference unit while the electron volt is an energy unit.
• Using the electron mass instead of its charge in the defining calculation; correct this by using the elementary charge 1.60 × 10⁻¹⁹ C.
• Forgetting to convert eV to J before substituting into an equation that requires SI units; correct this by multiplying the eV value by 1.60 × 10⁻¹⁹ J eV⁻¹ first.
• Multiplying when converting from J to eV; correct this by dividing the joule value by 1.60 × 10⁻¹⁹.
• Using 1.60 × 10⁻¹⁹ as a power of ten adjustment rather than as the charge factor; correct this by treating it as the conversion factor between the two energy units.
• Mixing up keV and eV by a factor of 1000; correct this by converting keV to eV first, then applying the joule conversion.
• Students often think that any amount of energy can be transferred to an atom, leading to continuous spectra. Correction: Only exact energy differences between levels are allowed, so spectra are discrete.
• Confusing excitation with ionisation. Correction: Excitation moves an electron to a higher bound level; ionisation removes it completely from the atom.
• Believing that the incident electron must have exactly the excitation energy to cause excitation. Correction: It must have at least that energy; any excess remains as kinetic energy of the incident electron.

1. 1Day 1-2: Revise the Bohr model and energy level diagrams. Learn the definitions of excitation, ionisation, and de-excitation.
2. 2Day 3-4: Study the conditions for excitation and ionisation in electron collisions. Practice calculations involving energy transfer and kinetic energy.
3. 3Day 5-6: Learn how line spectra are produced from electron transitions. Practice explaining the origin of emission and absorption spectra.
4. 4Day 7-8: Work through past paper questions on this topic, focusing on 4-6 mark explanations and calculations.
5. 5Day 9-10: Review common misconceptions and examiner tips. Create flashcards for key definitions and formulas.

• 📋Calculation of energy required for excitation or ionisation, often combined with kinetic energy of the incident electron. Advice: Convert units carefully and use conservation of energy.
• 📋Explanation of why a line spectrum is produced rather than a continuous spectrum. Advice: Link discrete energy levels to specific photon energies.
• 📋Determination of whether a collision is elastic or inelastic based on energy transfer. Advice: Compare incident electron energy to energy level differences.
• 📋Calculation of photon wavelength or frequency from an energy level transition. Advice: Use E = hf and c = fλ, and ensure correct unit conversions.